Regulations on the exhaust gases emitted from internal combustion engines such as a diesel engine have been tightened each year. In response to the tightening of these regulations, exhaust gas filters and the technology relating to the catalysts used with the filters are also making rapid progress. For example, techniques for reducing the particulate matter (PM) included in diesel engine exhaust gases have already been developed, including the use of a diesel particulate filter (DPF).
The DPF traps the PM included in the exhaust gases emitted from a diesel engine, and thus reduces the amount of PM emitted into the atmosphere. The DPF burns and removes trapped PM deposits to recover (regenerate) the PM-trapping ability of the filter. Usually, the PM deposited in the DPF is exposed to exhaust gases of a temperature higher than a fixed level and removed by burning (this process is referred to as natural regeneration of the filter). However, when the load upon the diesel engine is light, since the temperature of the exhaust gases does not reach the high temperature level required for the combustion of the PM, the natural regeneration of the DPF does not occur and if this DPF state remains unimproved, excessive PM deposition will clog the DPF.
To prevent this situation, it is necessary, for example, to burn and remove the trapped PM by forcibly increasing the temperature of the exhaust gases at fixed time intervals, or to calculate the clogging level from the differential pressure across the DPF and if the calculated clogging level exceeds a predetermined level, forcibly burn and remove the PM.
Patent Document 1, for example, describes a technique for detecting a differential pressure across a DPF by supplying pressures from the upstream and downstream sides of the DPF through pressure-introducing lines to one differential pressure sensor and detecting the differential pressure across the DPF. Patent Document 2 describes a technique for detecting a differential pressure across a DPF by detecting pressures by means of pressure sensors provided at the upstream and downstream sides of the DPF, and taking the difference.    Patent Document 1: JP, A 2005-344619    Patent Document 2: JP, A 7-317529